perf: Num fast path for std.member array search

Motivation:
std.member on arrays called ev.equal() for every element, incurring
reference equality check + isInstanceOf[Val.Func] + full pattern match
dispatch per comparison. For numeric arrays (the common case in
benchmarks like go_suite/member.jsonnet), this is pure overhead.

Modification:
- Hoist x.value outside the loop to avoid repeated Eval forcing
- Add Num fast path: when the search target is Val.Num, compare doubles
  directly via isInstanceOf[Val.Num] + asDouble, bypassing equal()
- General path unchanged for non-Num targets

Result:
member benchmark: from 1.97x slower to 1.04x faster than jrsonnet.
All 80 test suites pass. Collateral improvement on benchmarks that use
array equality internally (manifestJsonEx 2.51x→1.51x, setDiff
1.70x→1.43x, parseInt 1.36x→1.14x).

Author: He-Pin

sjsonnet/src/sjsonnet/stdlib/ArrayModule.scala

@@ -585,13 +585,30 @@ object ArrayModule extends AbstractFunctionModule {
             }
             str.str.contains(secondArg)
           case a: Val.Arr =>
-            var i = 0
-            var found = false
-            while (i < a.length && !found) {
-              if (ev.equal(a.value(i), x.value)) found = true
-              i += 1
+            val target = x.value
+            val len = a.length
+            // Num fast path: compare doubles directly, avoiding equal() dispatch
+            // (ref equality check + isInstanceOf[Val.Func] + pattern match chain)
+            target match {
+              case Val.Num(_, targetD) =>
+                var i = 0
+                var found = false
+                while (i < len && !found) {
+                  val elem = a.value(i)
+                  if (elem.isInstanceOf[Val.Num] && elem.asInstanceOf[Val.Num].asDouble == targetD)
+                    found = true
+                  i += 1
+                }
+                found
+              case _ =>
+                var i = 0
+                var found = false
+                while (i < len && !found) {
+                  if (ev.equal(a.value(i), target)) found = true
+                  i += 1
+                }
+                found
             }
-            found
           case arr =>
             Error.fail(
               "first argument must be an array or a string, got " + arr.prettyName